Michael Vaughan last night delivered the most forthright criticism of his side since taking over as England captain after they slumped to a worrying seven-wicket defeat against South Africa A.

The tourists plunged to the humiliating loss just four days before the first Test against the elite of South African cricket in Port Elizabeth, prompting Vaughan to accuse his players of "a little bit of complacency".

Several of the leading batsmen and bowlers looked desperately short of match practice and form.

Vaughan warned they could struggle in the forthcoming series if they fail to bounce back from such a performance.

England have won 10 out of the last 11 Tests to climb up to second place in the world rankings above South Africa, and Vaughan is demanding a drastic improvement.

"There are three days between now and the Test match starting on Friday and we'll have to make sure we're mentally right," stressed Vaughan, who compiled a second-innings century but still could not prevent the South African second-string cruising past their modest victory target of 135.

"We've got a lot of experience in the team and they're going to have to do a little talking and also we're going to have to rally around each other.

"There were spells in the game when we were very quiet for a team that's won 10 Tests out of the last 11 and maybe there was a little bit of complacency.

"I'm sure this will be a huge reminder that maybe we're not at the standard that everyone thinks we are, and if we play like this over the next few weeks we're going to get a hell of a surprise.

"We've got to make sure we up our game and make sure we're mentally prepared for Friday."

The captain dismissed claims that one warm-up match was not enough, particularly for the Test specialists who missed the one-day trips to Namibia and Zimbabwe. Instead he insisted that every player should now be ready to walk out for the first Test at St George's Park.

"The guys who came from Zimbabwe had the added time and the extra cricket, but that's certainly not an excuse - the guys have had time at home to get fit and go indoors, but I don't think they were switched off," said Vaughan.

"Maybe we under-estimated the opposition, maybe this was just what the team needed, a little bit of a kick up the backside and we'll see on Friday."

Having resumed overnight on 154 for seven, leading by just 98, England's last three wickets fell for 26 runs in five overs.

They were dismissed for 190 and the South Africans completed their shock triumph in only 30 overs, disrupted by lightning and rain.

England's players later volunteered for an extra practice session in the middle and Vaughan admitted: "It's not the best time to lose, but it can also be a good time.

"If there is a good thing about losing it can be the kick up the backside a team needs when maybe you're getting just a little bit too clever.

"I think we've probably tried to be too clever in this game and maybe we didn't take the game as hard as we should have done because the result suggests we got a hammering."

It was England's first defeat in a first-class match since losing the final Test against Sri Lanka in Colombo almost a year ago.

"What we tried to get out of the game was preventing England from spending much time out in the middle and they are going to have to find their feet quickly before this week's Test," said assistant coach Vincent Barnes. "Hopefully this will have an impact on the Test, but our full team have just come back from India. We played a warm-up game and nearly got beaten by the A side and when we took the field in the Test it was totally different - we played a hard game.

"I expect a pumped-up England side to come out on Friday.

"I'm not sure they took it lightly but I was surprised how out of synch some of the batters were and I was surprised by the margin of victory."